Cardiac pacemaker cable lead

ABSTRACT

A lead assembly adapted for endocardial fixation to a human heart is provided. The lead assembly includes a lead body that has a proximal end provided with a connector for electrical connection to a cardiac stimulator. The cardiac stimulator may be a pacemaker, a cardioverter/defibrillator, or a sensing instrument. The distal end of the lead body is connected to a tubular electrode housing. The lead body consists of a noncoiled conductor cable surrounded by a coextensive insulating sleeve. In contrast to conventional leads, the lead body of the present invention does not require coiled conductor wires or an internal lumen. Manipulation of the lead body is via an external guide tube. Lead body diameters of 0.25 mm or smaller are possible.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to cardiac stimulation leads, and moreparticularly to an implantable cardiac stimulation lead which employs alead body encasing a very thin noncoiled conductor cable.

2. Description of the Related Art

Prior to the advent of implantable endocardial stimulation leads,surgeons and cardiologists possessed few options for providing permanentor semi-permanent electro-physiological therapy to patients sufferingfrom cardiac arrhythmia. In cases where drug therapy and correctivesurgery were ruled out, epicardial leads used with external, and laterimplantable, pulse generators represented the normal clinical approach.For many patients whose arrhythmia stemmed from disruptions inelectrical signal propagation at highly localized spots deep within theheart, epicardial stimulation constituted a compromise treatment.

The introduction of endocardial leads capable of transvenousimplantation created a boon for many cardiac arrhythmia patients. Manyindividuals who formerly faced the prospects of median sternotomy orthoracotomy and reliance on epicardial stimulation for endocardiallyoriginated malfunctions could be provided with a subcutaneouslyimplanted cardiac stimulator combined with a transvenous lead thatpromised to yield better cosmetic results as well as the potential forbetter therapy through more accurate placement of lead electrodes.

Despite the myriad of advantages associated with endocardial leads,there has always been a tradeoff associated with their usage in manypatients. On the one hand, transvenous implantable leads typically yieldbetter cosmetic results and the potential for more accurate arrhythmiatherapy for patients. On the other, like any foreign body introducedinto the cardiovascular system, a transvenous cardiac lead presents anobstruction to the normal flow of blood, and possibly the normaloperation of one or more of the valves of the heart. This partialocclusion of a portion of the patient's cardiovascular system may resultin not only a diminished blood flow, but also may lead to the formationof microemboli.

For the majority of patients, the medical advantages associated withendocardial leads strongly outweigh the attendant obstruction to normalblood flow. However, for some patients, the calculation is less clear.Pediatric patients often present blood vessels that are simply too smallto accommodate conventional implantable leads, and these young patientsare often the least able to adjust successfully to a diminished bloodflow and/or valve function. Similarly, those patients who presentoccluded vessels and/or eroded valve leaves resulting from disease,injury, or some other mechanism may not be suitable candidates fortransvenous implanted leads. In these types of cases, epicardial leadsmay present the only viable solution for the arrhythmia patient.

The magnitude of blood flow area of a given vessel obstructed by aconventional endocardial lead is a function of the diameter of the leadbody. Early designs for endocardial leads consisted of an elongated leadbody that included a proximal connector for connection to a pulsegenerator and a distally located electrode for transmitting signals tothe heart. The lead body consisted of a tubular insulating sleeve thatjacketed a coiled conductor wire leading from the electrode to theconnector. The conductor wire was coiled in a helical fashion to leave acentrally disposed lumen through which a stylet could be inserted tomanipulate the lead. The minimum overall diameter for this design islimited by the sum of the diameter of the lumen, twice the diameter ofthe conductor wire, and twice the wall thickness of the sleeve. An earlybipolar variant incorporated two coiled conductor wires separatelydisposed in respective lumens. Here, the minimum diameter is a functionof the sum of the diameters of both lumens, twice the diameter of theconductor wire, and twice the thickness of the sleeve. Diameters of 8French (approximately 2.7 mm) (1 French=3×diameter in millimeters)) werenot uncommon.

Later lead designs incorporated a coaxial arrangement that representedan advance in miniaturization. The coaxial lead utilizes a lead bodywith an inner conductor wire defining a lumen, an outer conductor wire,an intermediary insulating sleeve separating the two conductor wires,and an outer insulating sleeve. The minimum diameter of the coaxialbipolar lead body is limited by the sum of the diameters of the lumen,the first conductor coil, the intermediary insulator sleeve, the secondconductor coil, and the outer sleeve. Overall diameters of about 6French (approximately 2 mm) are common with this design.

A recent improvement upon the coaxial bipolar design incorporates nestedand individually insulated conductor wires that circumscribe aconcentrically located lumen. This uniaxial design can be seen in theThinline™ (a trademark of Sulzer Intermedics, Inc.) leads produced bySulzer Intermedics, Inc. The diameter of the Thinline™ lead body is afunction of the sum of the diameter of the lumen, the diameter of eachof the conductor wires, and twice the wall thickness of the outersleeve. The introduction of the Thinline™ lead design further reducedthe minimum diameter of the lead body to about 4.7 French (approximately1.6 mm).

Despite advances in miniaturization, there are still severaldisadvantages associated with conventional lead designs. Conventionallead bodies require an internal lumen that is coextensive with the leadbody to accommodate an internal stylet for manipulating the lead. Thediameter of the lumen often constitutes a significant portion of theoverall diameter of the lead body and therefore represents a limitationon the achievable miniaturization of the lead body. Similarly,conventional lead bodies incorporate coiled conductor wires that, bydefinition, contribute twice their own diameters to the overall diameterof the lead body. For these reasons the smallest available conventionalleads may still be too large for successful transvenous implantation insome patients.

In addition, coaxial leads are susceptible to structural failure due toa phenomenon commonly known as "subclavian crush." Subclavian crushoccurs when a lead is implanted via the subclavian vein (a commontransvenous entry site) and is pressed against the patient's clavicleduring movement of the shoulder joint. The pressing force may bend thecoils of the lead wire to fracture. The problem is exacerbated if thepatient suffers an externally applied trauma in the clavicle area.

The present invention is directed to overcoming or minimizing one ormore of the foregoing disadvantages.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a lead assemblyis provided. The lead assembly includes a tubular electrode housing thathas a proximal end, a fixation mechanism, a first electrode and a secondelectrode. A lead body is provided that has a first end coupled to theproximal end of the tubular electrode housing, a second end, a firstnoncoiled conductor cable coupled to the first electrode, a secondnoncoiled conductor cable disposed in parallel relation to the firstnoncoiled conductor cable and coupled to the second electrode, and aninsulative sleeve coating the first and second noncoiled conductorcables. A connector is provided that has a distal end coupled to thelead body for coupling to a cardiac stimulator.

In accordance with another aspect of the present invention, a leadassembly is provided. The lead assembly includes a connector forcoupling to a cardiac stimulator and a lead body. The lead body has afirst end coupled to the connector, a second end, an elongated noncoiledconductor cable, and an insulative sleeve coating the noncoiledconductor cable. A tubular electrode housing is provided that has aproximal end coupled to the second end of the lead body, a fixationmechanism, an electrode, and a lumen extending through the tubularelectrode housing. The lumen is eccentrically disposed relative to thesecond end of the conductor cable to enable the tubular electrodehousing to slidably engage a stylet temporarily implanted to a desiredlocation in advance of the lead assembly.

In accordance with still another aspect of the present invention, a leadassembly is provided. The lead assembly includes a connector forcoupling to a cardiac stimulator and a lead body. The lead body has afirst end coupled to the connector, a second end, an elongated noncoiledconductor cable, and an insulative sleeve coating the conductor cable.The lead body has a diameter smaller than about 4.7 French. A tubularelectrode housing is provided that has a proximal end coupled to thesecond end of the conductor cable, a fixation mechanism, an electrode,and a lumen extending through the tubular electrode housing. The lumenis eccentrically disposed relative to the second end of the lead body toenable the tubular electrode housing to slidably engage a stylettemporarily implanted to a desired location in advance of the leadassembly. An elongated guide tube is provided that has a distal endremovably engageable with the proximal end of the tubular electrodehousing.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a pictorial view of an exemplary embodiment of a lead assemblyin accordance with the present invention;

FIG. 2 is a cross-sectional view of FIG. 1 taken at section 2--2;

FIG. 3 is the same view as FIG. 2 drawn approximately to a particularscale;

FIG. 4 is a cross-sectional view of a conventional Thinline™ lead bodydrawn to the same scale as FIG. 3;

FIG. 5 is a cross-sectional view of FIG. 1 taken at section 5--5;

FIG. 6 is an exploded pictorial view of a portion of the electrodehousing depicted in FIG. 5 in accordance with the present invention;

FIG. 7 is a cross-sectional view like FIG. 5 of an alternate embodimentof the electrode housing in accordance with the present invention;

FIG. 8 is a cross-sectional view like FIG. 5 of another alternateembodiment of the electrode housing in accordance with the presentinvention;

FIG. 9 is the pictorial view of FIG. 1 showing the placement of theguide tube in accordance with the present invention;

FIG. 10 is a cross-sectional view of FIG. 9 taken at section 10--10;

FIG. 11 is a pictorial view of an alternate embodiment of the guide tubeshown in FIG. 9 in accordance with the present invention;

FIG. 12 is a pictorial view of another alternate embodiment of the guidetube shown in FIG. 9 in accordance with the present invention;

FIG. 13 is a cross-sectional view like FIG. 5 showing an alternateembodiment of the electrode housing configured to slide over a stylet inaccordance with the present invention;

FIG. 14 is a cross-sectional view like FIG. 5 showing an alternateembodiment of the lead assembly that incorporates a bipolar lead body inaccordance with the present invention; and

FIG. 15 is a cross-sectional view like FIG. 2 showing the lead body ofFIG. 14 in accordance with the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the drawings described below, reference numerals are generallyrepeated where identical elements appear in more than one figure.Turning now to the drawings, and in particular to FIGS. 1 and 2, thereis shown an exemplary lead assembly 10 that is adapted for endocardialfixation to a human heart. The lead assembly 10 includes a lead body 12that has a proximal end 14 provided with a connector 16 for electricalconnection to a cardiac stimulator 18. The cardiac stimulator 18 may bea pacemaker, a cardioverter/defibrillator, or a sensing instrument. Thedistal end 20 of the lead body 12 is connected to a tubular electrodehousing 22. The proximal end 14 of the lead body 12 may be coupled tothe connector 16 by conventional means such as crimping, laser, or spotwelding.

FIG. 2 is a highly exaggerated cross sectional view of the lead body 12taken at section 2--2. The view is exaggerated in scale because theactual diameter of the lead body 12 is only somewhat larger than a humanhair. The lead body 12 consists of a conductor cable 24 surrounded by acoextensive insulating sleeve 26. The conductor cable 24 may be a singlefilament wire or a plurality of individual conductor wires 28 as shownThe precise number and arrangement of the conductor wires 28 is a matterof design discretion. In the embodiment shown, the conductor cable 24consists of nineteen individual metal conductor wires 28 having acombined diameter of approximately 0.127 mm. The insulating sleeve 26has a wall thickness of approximately 0.0508 mm, making the totaldiameter of the lead body approximately 0.229 mm or 0.69 French. Incontrast to conventional lead bodies, the conductor cable 24 isnoncoiled, that is, not spiraled to define a concentrically disposedlumen. Consequently, the minimum diameter of the lead body 12 is limitedonly by the sum of the diameter of the cable 24 and twice the wallthickness of the sleeve 26.

The vivid contrast between the lead body 12 and a conventional 4.7French diameter Thinline™ lead body can be readily seen in FIGS. 3 and4, which show, respectively, side-by-side cross sectional views of thelead body 12 and a conventional Thinline™ lead body 30 both drawn to thesame 1/32"=0.001" scale. FIGS. 3 and 4 show the relative sizedifferences between the lead body 12 of the present invention and aconventional Thinline™ lead body 30. The conventional lead body 30consists of coiled and nested conductor wires 32 and 34 defining aconcentrically disposed lumen 36. The conductor wires 32 and 34 aresurrounded by an insulating sleeve 38. The total diameter of the leadbody 30 is a combination of twice the wall thickness of the sleeve 38,the combined diameters of the conductor wires 32 and 34, and theconcentrically disposed lumen 36.

The conductor cable 24 is preferably manufactured from a biocompatibleconducting material, such as, for example, MP35N alloy. MP35N alloygenerally consists of a combination of cobalt, chromium, nickel, andmolybdenum. A further discussion of the properties of MP35N alloy may behad by reference to U.S. Pat. Nos. 3,356,542 and 3,562,024. The leadbody 12 should be capable of readily conforming to the irregularpassageways and shapes of the cardiovascular system. Accordingly, theconductor cable 24 should have a high enough ductility to permit thelead body 12 to flex easily, and elastically. The conductor cable 24 isnormally cold worked during fabrication. In the event the conductorcable 24 is composed of several individual wires 28, it is anticipatedthat the wires 28 should be slightly twisted to keep them together priorto the application of the sleeve 26. However, the wires 28 may have atendency to resist the twist and spring apart due to the previous coldwork. In this regard, the wires 28 may be heat set so that they do notunfurl prior to the application of the sleeve 26. A variety of heatsetting protocols may be suitable. One possibility involves tempering at600° F. for approximately one hour in an inert ambient, such as argon.The fully fabricated cable 24 may be obtained from the Xylem Company inWayzata, Minn.

The insulating sleeve 26 is designed to provide biocompatible electricalinsulation for the conductor cable 24 while providing an externalsurface that is smooth and does not promote microemboli. The sleeve 26is preferably fabricated from a biocompatible polymer material, such as,for example, ETFE (fluoropolymer resin), or a similar biocompatiblepolymer material.

The detailed structure of the electrode housing 22 may be understood byreference to FIGS. 1 and 5. FIG. 5 is a cross sectional view of FIG. 1taken at section 5--5. The electrode housing 22 includes a tubularelectrode member 40 that has a proximally projecting reduced diameterportion 42 which defines a proximally projecting annular shoulder 44located near the distal end of the tubular electrode member 40. Thereduced diameter portion 42 is surrounded by an insulating sleeve 46that is composed of a conventional biocompatible material such aspolyurethane or silicone rubber. The distal end of the insulating sleeve46 abuts against the annular shoulder 44. The proximal end of theinsulating sleeve 46 includes an opening 48 and the distal end of thetubular electrode member 40 includes an opening 50. The interiorsurfaces of the tubular electrode member 40 and the insulating sleeve 46as well as the openings 48 and 50 define a lumen 52. The proximal end ofthe insulating sleeve 46 includes a reduced diameter portion 54 that hasa longitudinally disposed slot 56, the function of which is disclosedbelow.

A semi tubular plug 58 is disposed inside the tubular electrode member40. The central portion 60 of the plug 58 includes a cylindrical surfacethat is sized to provide an interference fit with the interior surfaceof the tubular electrode member 40. The distal portion of the plug 58includes a reduced diameter cylindrical tip 62. A fixation mechanism orcorkscrew 64 is coiled around the exterior of the tip 62. The distal endof the corkscrew 64 projects from the opening 50 to provide activefixation to endocardial tissue. The plug 58 includes a lumen 66 thatextends from the proximal end of the plug 58 to the distal end of thetip 62. However, note that the distal end of the tip 62 is closed. Thelumen 66 is designed to receive a stylet under certain circumstances asdiscussed more fully below.

The upper side of the proximal portion of the plug 58, as viewed in FIG.5, includes a cut-out 68, the structure and function of which may beunderstood by referring now also to FIG. 6, which is an explodedpictorial view of the plug 58 removed from the electrode housing 22. Thehorizontal surface 70 of the cut out 68 provides a platform upon whichthe distal end 72 of the lead body 12 may be secured. The distal end 72of the lead body 12 is disposed on the horizontal surface 70 in aserpentine-like fashion. Most of the insulating sleeve 26 should beremoved from the distal portion 72 to expose the conductor cable 24,though some portion of the sleeve 26 should be left on the distal end 72in the vicinity of the proximal edge of the cut-out 68 to reduce thepotential for short circuiting caused by body fluids. The particularconfiguration of the serpentine-like arrangement is a matter ofdiscretion. However, care should be taken to provide the first bend 73in the distal end 72 with a relatively large radius to reduce thepotential for a stress riser.

A crimp block 74 is provided to secure the distal end 72 of the leadbody 12 to the plug 58. The crimp block 74 is dimensioned to correspondto the cut out 68, and when pressed tightly on the cut-out 68 and overthe distal end 72, acts as a crimping member to hold the distal end 72in place as shown in FIG. 5. The crimp block 74 may be suitablydimensioned and/or the tubular electrode 40 may be swaged to provide aninterference fit between the crimp block 74 and the interior surface ofthe reduced diameter portion 42 of the tubular electrode member 40. Inaddition to relying on friction to secure the lead body 12 to theelectrode housing 22, the conductor cable 24 may also be spot or laserwelded to the horizontal surface 70 to provide an additional attachmentmechanism in the event the plug 58 is fabricated from a weldablematerial.

The fixation mechanism or corkscrew 64 may be replaced by a passivefixation mechanism to secure the electrode housing 22 to endocardialtissue. FIG. 7 is a view of similar perspective to FIG. 5 and shows anembodiment of the electrode housing, now designated 22', that includesone or more outwardly projecting tines 75 that provide passive fixation.The number and arrangement of the tines 75 is a matter of designdiscretion. The tines 75 may be composed of a non-metallic biocompatiblematerial, such as, for example, silicone rubber, polyurethane,polyethylene, polyimide, or similar materials.

The corkscrew 64 and the tubular electrode member 40 may be fabricatedfrom a variety of biocompatible conducting materials, such as, forexample, iridium oxide coated titanium. Other possible materials includeMP35N, stainless steel, platinum-iridium alloy consisting ofapproximately 90% platinum and 10% iridium, or some other biocompatibleconducting metal. The corkscrew 64 is preferably coated with a thincoating of an insulating polymer, such as Parylene C® supplied by UnionCarbide, or a similar material. In general, the plug 58 and crimp block74 may be fabricated from the same types of materials as the corkscrew64, or may be composed of a non-metallic, biocompatible material, suchas, for example, silicone rubber, polyurethane, polyethylene, polyimide,or similar materials. If the plug 58 is composed of a metallic material,the corkscrew 64 may be secured to the tip 62 by spot or laser welding.However, an electrical pathway must be established between the distalend 72 of the lead body 12 and the tubular electrode member 40. If theplug 58 is fabricated from a metallic material, this pathway is providedby the plug 58 itself. In this circumstance, the crimp block 74 need notbe composed of a conducting material and may instead be fabricated froma variety of biocompatible, nonconducting materials, such as siliconerubber, polyurethane, polyethylene, polyimide, or similar materials.However, if the plug 58 is fabricated from a nonconducting material, thecrimp block 74 should be fabricated from the same types of materials asthe corkscrew 64 to establish the requisite electrical pathway from thedistal end 72 to the tubular electrode member 40.

The particular mechanism for securing the lead body 12 to the electrodehousing 22 may take on a variety of configurations. For example, FIG. 8shows an alternate embodiment of the electrode housing 22, nowdesignated 22". In this embodiment, the plug 58 depicted in FIGS. 5 and6 is replaced with a more streamlined plug 58' that is of relativelyuniform diameter, and acts as a conventional crimp slug. The reduceddiameter portion 42 of the tubular electrode member 40 is configured toserve as a crimp sleeve to secure the distal end 72. Note that in thisembodiment, the distal end 72 is looped one or more times around theplug 58'. As in the foregoing embodiment, the distal end 72 may be laseror spot welded to the plug 58' prior to crimping by the tubularelectrode member 40.

The manipulation of the lead assembly 10 during implantation orextraction may be understood by reference to FIGS. 9 and 10, which show,respectively, a pictorial view of the lead assembly 10 and a crosssectional view of FIG. 9 taken at section 10--10. A guide tube 76 may beslipped over the proximal end 54 of the electrode housing 22 tospatially manipulate the lead assembly 10. The guide tube 76 is providedwith a proximally disposed handle 78 for manipulation of the guide tube76 by hand. The distal end 80 of the guide tube 76 is provided with alongitudinally projecting key 82 that is configured to engage the slot56 in the electrode housing 22. The engagement of the key 82 and theslot 56 enable a torque applied to the handle 78 to transmit into arotational movement of the electrode housing 22, and in this way enablethe corkscrew 64 to be twisted into the endocardium 83. The guide tube76 does not require a key 82 if the electrode housing 22 utilizespassive fixation. The distal end 80 is also provided with alongitudinally aligned slot 84 that is peripherally displaced from thekey 82. The presence of the slot 84 enables the guide tube 76 to beslipped over the reduced diameter portion 54 of the electrode housing 22while maintaining the lead body 12 external to the guide tube 76. Toprevent the guide tube 76 from slipping off of the electrode housing 22,the distal end 80 may be dimensioned to provide an interference fit withthe reduced diameter portion 54 of the electrode housing 22, or tensionmay be maintained on the lead body 12 after the distal end 80 is slippedover the electrode housing 22 and until the electrode housing 22 issecured to the endocardium 83.

The guide tube 76 functions as a stylet. Accordingly, it is desirablefor the guide tube 76 to be fabricated from a ductile material that maybe readily plastically deformed by hand so that the surgeon can bend theguide tube 76 at various locations, as necessary, to place the electrodehousing 22 at the desired location within the heart. Exemplary materialsinclude nickel-titanium alloys commonly sold under the trade nameNitinol, polyurethane, polyethylene, and polyamide. These materials, andparticularly Nitinol, are resistant to kinking.

To disengage the guide tube 76 from the electrode housing 22, a stylet86 may be introduced into the guide tube 76 through the handle 78 andadvanced into the lumen 66. The guide tube 76 and the electrode housing22 may then be separated by simultaneously applying a thrust to thestylet 86 and a tensile force to the guide tube 76. The stylet 86 maythen be withdrawn.

The particular mechanism for transmitting torque from the guide tube 76to the electrode housing 22 may be varied. FIGS. 11 and 12 show,respectively, pictorial views of two such alternative configurations. InFIG. 11, the guide tube, now designated 76', is provided with twodiametrically opposed radially projecting cylindrical keys 88, only oneof which is shown in FIG. 11. The electrode housing, now designated22'", is provided with a pair of diametrically opposed arcuate slots 90,only one of which is shown in FIG. 11. The guide tube 76' is dimensionedto be slightly smaller in diameter than the electrode housing 22'" sothat the guide tube 76' may be slipped into the proximal end of theelectrode housing 22'" and advanced longitudinally with the keys 88engaging the corresponding slots 90. The interaction of the keys 88 andthe slots 90 enable transmission of torque from the guide tube 76' tothe electrode housing 22". FIG. 12 shows another alternativeconfiguration for the guide tube, now designated 76". In thisembodiment, the keys, now designated 88', are disposed internal to theguide tube 76' and the guide tube 76" is dimensioned to be slightlylarger in diameter than the electrode housing 22'" so that the guidetube 76" may be slipped over the exterior of the electrode housing 22'"and the keys 88 brought into sliding longitudinal engagement with theslots 90. Torque may then be transmitted from the guide tube 76" to theelectrode housing 22'". The keys 88 and 88' may be formed from the samematerials used to fabricate the guide tubes 76 or 76" and may beintegrally formed with the guide tubes 76 or 76" or fabricated asseparate pins welded or press fit to the guide tube 76 or 76".

The implantation procedure of the lead assembly 10 may be understood byreference to FIGS. 9 and 10. The guide tube 76 is secured to theelectrode housing as discussed above. The stylet 86 may also be insertedat this point. The electrode housing 22 is then introduced into one ofthe major veins leading to the heart, such as the subclavian vein or oneof the internal jugular veins. Following initial transvenous entry, theelectrode housing 22 is advanced by manipulation of the guide tube 76and/or the stylet 86 until the electrode housing 22 is located at thedesired point of fixation to the endocardium 83. If active fixation isemployed, the surgeon may then twist the handle 78 to engage the corkscrew 64 with the endocardium 83. If not, the surgeon need not twist thehandle 78. In either case, the stylet 86 may then be used as necessaryto disengage the guide tube 76 from the electrode housing 22. The guidetube 76, and the stylet 86, if used, may then be retracted and theconnector 16 may then be connected to the cardiac stimulator 18. If theinitial placement is unsatisfactory, the procedure may be reversed andrepeated as often as necessary.

FIG. 13 is a partial sectional view of an alternate embodiment of thelead assembly 10 taken at the same general section as FIG. 5. In thisembodiment, a particular pathway to the desired fixation point on theendocardium 83 is preestablished using a stylet 92. The tip 62 of theplug 58 is provided with an opening 94 so that the electrode housing,now designated 22"", may be slipped over the proximal end of the stylet92. Any of the guide tubes disclosed above may then be secured to thereduced diameter portion 54 of the electrode housing 22"" and then, bymanipulation of the guide tube 76, the electrode housing 22 may beadvanced along the stylet 92 until the electrode housing 22"" isdisposed near the desired fixation point as shown in FIG. 13. Thecorkscrew 64 may then be secured to the endocardium 83 as disclosedabove, either with the stylet 92 still in place or after the stylet 92has been withdrawn. After the corkscrew 64 has been secured to theendocardium 83, the stylet 92 and the guide tube 76 (see FIG. 8) may beremoved.

If the guide tube 76 is provided with an interference fit with theelectrode housing 22, tension applied to the guide tube 76 to disengageit from the electrode housing 22 will be resisted only by the engagementof the corkscrew 64 with the endocardium 83. This may result in adisengagement of the corkscrew 64 with the endocardium 83. Accordingly,a stylet of the type shown in FIG. 8 but with a diameter sized toprovide a slight interference fit with the lumen 66 may be insertedfollowing removal of the stylet 92 to provide an opposing thrust againstwhich tension applied to the guide tube 76 may act to achieve thedesired disengagement. In this embodiment, the distal end 72 should beeccentrically disposed relative to the lumen 66 to avoid interferingwith the movement of the electrode housing 22"" along the stylet 92.

The foregoing embodiments incorporate a unipolar lead body 12. However,multipolar lead bodies may be alternatively provided to facilitatemultipolar stimulation and/or sensing. FIG. 14 is a cross-sectional viewlike FIG. 5 of an alternate embodiment of the lead assembly, nowdesignated 10', which employs a bipolar lead body, now designated 12'.The lead body 12' is bifurcated into two forks 96a and 96b at the branch98. FIG. 15 is cross-sectional view of the lead body 12' taken proximalto the branch 98. As shown in FIG. 15, the lead body 12' includes twononcoiled conductor cables 24' and 24" disposed in a parallelrelationship and surrounded by an insulating sleeve 26'. Distal to thebranch 98, each of the cables 24' and 24" is individually covered by aportion of the sleeve 26' and is similar in cross-section to the leadbody 12 shown in FIG. 5. The branch 98 may be formed by severing thesleeve 26' between the cables 24' and 24" or by separately jacketing thedistal ends of the cables 24' and 24" at the time the lead body 12' isfabricated. The sleeve 26' and the cables 24' and 24" may be fabricatedfrom the materials disclosed above.

The branch 96a is coupled to the electrode housing, now designated22'"", in the manner described above for coupling the lead body 12. Theother branch 96b is connected to a second annular electrode 100 that isdisposed over the sleeve 46' proximal to the electrode housing 22'"".The sleeve 46' is elongated proximally to accommodate the electrode 100.Two annular members 102 and 104 are disposed between the sleeve 46' andthe second electrode 100. To establish electrical connection between thebranch 96b and the electrode 100, the branch 96b is projected through anopening 106 in the sleeve 46' located in that portion of the sleeve 26'covered by the annular electrode 100. The distal end of the branch 96bis stripped to expose the bare cable 24". The bare cable 24" issandwiched between the exterior of the annular member 104 and theinterior of the annular electrode 100. Prior to installing the annularelectrode 100, the bare cable 24" is secured to the annular member 104by laser or spot welding. After the cable 24" is secured to the annularmember 104, the annular electrode 100 is positioned and swaged. Theswaging serves to reduce the diameter of the annular electrode 100 andto ensure physical contact between the annular electrode 100 and thecable 24" and/or the annular member 104.

The skilled artisan will appreciate that several electrodes may beincorporated into the lead assembly 10' and serviced by a correspondingplurality of noncoiled cables disposed in a parallel relationship. Theoverall diameter of such a lead body will be significantly smaller thana conventional lead body. Regardless of the number of cables in the leadbody 12', the implantation procedure will be the same as disclosedabove.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A lead assembly, comprising:a tubular electrodehousing having a proximal end, a fixation mechanism, a first electrodeand a second electrode; a lead body having a first end coupled to theproximal end of the tubular electrode housing, a second end, a firstnoncoiled conductor cable coupled to the first electrode, a secondnoncoiled conductor cable disposed in parallel relation to the firstnoncoiled conductor cable and coupled to the second electrode, and aninsulative sleeve coating the first and second noncoiled conductorcables; and a connector having a distal end coupled to the lead body forcoupling to a cardiac stimulator.
 2. The lead assembly of claim 1,wherein the lead body has a diameter smaller than about 4.7 French. 3.The lead assembly of claim 1, wherein the fixation mechanism comprises acorkscrew.
 4. The lead assembly of claim 1, wherein the fixationmechanism comprises a tine projecting outwardly from the tubularelectrode housing.
 5. The lead assembly of claim 1, comprising anelongated guide tube having a distal end removably engageable with theproximal end of the tubular electrode housing.
 6. The lead assembly ofclaim 5, wherein the proximal end of the tubular electrode housing has aslot and the distal end of the guide tube has a key to cooperativelyengage the slot.
 7. The lead assembly of claim 1, wherein the tubularelectrode housing has a lumen extending therethrough and beingeccentrically disposed relative to the first end of the conductor cableto enable the tubular electrode housing to slidably engage a removablestylet temporarily implanted to a desired location in advance of thelead assembly.
 8. A lead assembly, comprising:a connector for couplingto a cardiac stimulator; a lead body having a first end coupled to theconnector, a second end, an elongated noncoiled conductor cable, and aninsulative sleeve coating the conductor cable; and a tubular electrodehousing having a proximal end coupled to the second end of the conductorcable, a fixation mechanism, an electrode, and a lumen extending throughthe tubular electrode housing, the lumen being eccentrically disposedrelative to the second end of the lead body to enable the tubularelectrode housing to slidably engage a stylet temporarily implanted to adesired location in advance of the lead assembly.
 9. The lead assemblyof claim 8, wherein the lead body has a second noncoiled conductor cabledisposed in parallel relation to the first noncoiled conductor cable,the second noncoiled conductor cable having a second electrode couplethereto.
 10. The lead assembly of claim 8, wherein the lead body has adiameter smaller than about 4.7 French.
 11. The lead assembly of claim8, wherein the fixation mechanism comprises a corkscrew.
 12. The leadassembly of claim 8, wherein the fixation mechanism comprises a tineprojecting outwardly from the tubular electrode housing.
 13. The leadassembly of claim 8, comprising an elongated guide tube having a distalend removably engageable with the proximal end of the tubular electrodehousing.
 14. The lead assembly of claim 13, wherein the proximal end ofthe tubular electrode housing has a slot and the distal end of the guidetube has a key to cooperatively engage the slot.
 15. A lead assembly,comprising:a connector for coupling to a cardiac stimulator; a lead bodyhaving a first end coupled to the connector, a second end, an elongatednoncoiled conductor cable, and an insulative sleeve coating theconductor cable, the lead body having a diameter smaller than about 4.7French; a tubular electrode housing having a proximal end coupled to thesecond end of the conductor cable, a fixation mechanism, an electrode,and a lumen extending through the tubular electrode housing, the lumenbeing eccentrically disposed relative to the lead body to enable thetubular electrode housing to slidably engage a stylet temporarilyimplanted to a desired location in advance of the lead assembly; and anelongated guide tube having a distal end removably engageable with theproximal end of the tubular electrode housing whereby.
 16. The leadassembly of claim 15, wherein the lead body has a second noncoiledconductor cable disposed in parallel relation to the first noncoiledconductor cable, the second noncoiled conductor cable having a secondelectrode couple thereto.
 17. The lead assembly of claim 15, wherein thefixation mechanism comprises a corkscrew.
 18. The lead assembly of claim15, wherein the fixation mechanism comprises a tine projecting outwardlyfrom the tubular electrode housing.
 19. The lead assembly of claim 15,wherein the proximal end of the tubular electrode housing has a slot andthe distal end of the guide tube has a key to cooperatively engage theslot.